In certain applications, it is advantageous to extrude plastic tubular products which are of superior strength and rigidity. This allows the plastic tubes to be used where the need for structural support is mandated. It is a purpose of this invention to increase the strength of common tubular plastic products without reliance on expensive, exotic materials, such as liquid crystal polymers, PTFE and like rigid chain polymers. To accomplish this purpose, this invention uses an extrusion die assembly for common thermoplastic materials to provide a rotary motion to the molten plastic material as it is extruded.
Several attempts have been made, in particular in the medical field, to increase strength and rigidity by orienting the extruded material in a helical manner. Such an attempt is illustrated in U.S. Pat. No. 5,156,785 which describes a method by which catheters are constructed from a mixture of liquid crystal polymers and other plastics. Using an extrusion die with rotating components, fibrils of liquid crystal polymer are oriented in a helical pattern in order to construct catheters with greater rotational stiffness. In U.S. Pat. No. 5,248,305, which is related to the ""785 patent, a schematic depiction of an extrusion head with rotating components is shown in FIG. 4. It should be noted that there is no description of how the relative rotation of the extrusion die elements, is accomplished. It is a purpose of this invention to construct a reliable system for rotating the opposing surfaces which form an extrusion channel in an extrusion die to preferentially bias the molecular structure of common thermoplastic materials.
The above patents, at least in part, were the result of work relating to improving self reinforcing composites in which the beneficial effects of liquid crystal polymers were employed. Liquid crystal polymers exhibit a rod like polymer chain which has been found to align more readily than flexible chain polymers when subjected to a shear force (D. G. Baird, et al, The In-Situ Generation of Liquid Crystalline Reinforcements in Engineering Thermoplastics). This reference seems to teach that flexible chain polymers have constructive results only when blended with liquid crystal polymers. Similarly, in the reference Farell, et al, A Rotating Annular Die To Control the Biaxial Orientation in Melt Processed Thermotropic Liquid Crystalline Cellulose Derivatives, only low levels of preferential orientation or a negative effect was experienced when polyethylene was subjected to shear by virtue of a rotating die head. All of the above systems refer to a single extruded layer.
A purpose of this invention is to provide a system for producing a product using common, flexible chain polymers which are applied in at least two layers having different orientations which enhance the strength characteristics of tubular plastic products. In the system shown in Japanese Patent Application No. 63-199622, Aug. 18, 1988, Moulding Method for Multilayer Film, a liquid crystal polymer is blended with a polyester resin in a channel formed by rotating components. No layers are formed as the materials are introduced side by side and therefore mixed in a single layer.
It is the purpose of this invention to combine an extrusion die assembly having a balanced flow passage with rotating die elements. A balanced flow extrusion die passage is described in U.S. Pat. No. 5,667,818, the disclosure of which is incorporated herein by reference.
Another purpose of this invention is to provide a tubular product constructed of multiple layers of extruded common thermoplastic polymers having different helical molecular structures to provide an enhanced strength tubular product.
A tubular product is constructed with multiple layers of extruded flexible chain polymer. Adjacent layers are subjected to different or opposing shear forces in order to generate differing helical molecular structures in the adjacent layers. The helixes thus formed will be at an angle to each other. The relatively transverse helixes will generate very strong tubular products.
To accomplish this-unique structure, the layers of flexible chain polymer material are joined and passed into a restricted annular chamber. The chamber is formed by inner and outer cylindrical surfaces, in which the outer surface is formed on a die module and the inner surface is formed by a tip module. By rotating the modules, these surfaces may be rotated in opposing directions or at a differing relative rotational speed depending on the performance specification desired for the tubular product. The shear forces exerted on the respective layers by the rotating surfaces will induce a spiraling orientation of the polymer in each layer which will be relatively different depending on the direction and speed of the surfaces. The helixes will be oriented at differing angles to a plane transverse to the axis of rotation, analogous to the pitch angle of a screw.
A convenient mechanism for providing the annular shear chamber is also shown in this application. A balanced flow passage is constructed in a series of axially aligned die elements to supply at least two layers of extruded plastic to the shear chamber. In the case of a two layer construction, there will be an inner and outer layer which are directly exposed to the differing shear forces of the rotating surfaces which form the shear chamber. Each of these surfaces are attached to external drive-motors which are independently controlled to provide relative motion between the chamber surfaces. The chamber surfaces are constructed on independent elements of the die assembly which are separately mounted for rotation within the assembly. Appropriate bearing assemblies are provided which can withstand the high temperature environment of the assembly. In addition a specialized sealing system is needed to prevent leakage of the extruded materials.